High modulus wear resistant gray cast iron for piston ring applications

ABSTRACT

A piston ring formed of cast iron provides improved machinability and exceptional performance and minimum costs. The cast iron includes 2.2 to 2.9 wt. % carbon, 3.2 to 4.2 wt. % silicon, 0.75 to 1.25 wt. % copper, 1.0 to 1.5 wt. % manganese, 0.09 to 0.15 wt. % sulfur, not greater than 0.2 wt. % phosphorous, and an average carbon equivalent of 3.8. The cast iron preferably includes a matrix of martensite with MnS and carbides dispersed therein. The matrix is also preferably free of ferrite, austenite, and steadite. The cast iron is formed by casting, autenitizing, quenching, and tempering the alloy.

This divisional application claims priority to U.S. Utility applicationSer. No. 13/688,802, filed Nov. 29, 2012 and U.S. ProvisionalApplication Ser. No. 61/565040, filed Nov. 30, 2011, both of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to piston rings for internal combustionengines, materials thereof, and methods of forming the same.

2. Description of the Prior Art

Piston rings of internal combustion engine applications are often formedof cast iron compositions providing wear resistance and seizureresistance. U.S. Pat. No. 5,972,128 to Miwa discloses cast ironcompositions used to form piston rings. The cast irons disclosed in Miwainclude, in wt. % of the cast iron, 3.15 to 3.16 wt. % carbon, 2.66-2.86wt. % silicon, 0.65 to 1.18 wt. % copper, 0.7 to 0.78 wt. % manganese,0.04 to 0.06 wt. % sulfur, and 0.10 to 0.12 wt. % phosphorous. U.S. Pat.No. 4,891,076 and U.S. Pat. No. 5,985,052 also disclose example castiron compositions.

One type of cast iron is referred to as gray cast iron, or gray iron.There are various grades of gray iron, and a typical gray ironcomposition includes, in weight percent (wt. %) of the gray iron, 2.0 to4.0 wt. % carbon, 1.25 to 3.25 wt. % silicon, 0.75 to 1.25 wt. %manganese, 0.08 to 0.12 wt. % sulfur, and 0.07 to 0.2 wt. % phosphorous,based on the total weight of the gray iron.

Recently, there has been increased demand for piston rings includingcomplex physical features, such as advanced oil groove designs, toprovide increased performance and reduce costs. However, the cast ironsof the prior art are oftentimes difficult to machine, or requireexpensive alloying additions to achieve the desired physical properties,which limits their use in piston rings of internal combustion engineapplications.

SUMMARY OF THE INVENTION

One aspect of the invention provides a cast iron including, in weightpercent (wt. %) of the cast iron, 2.2 to 2.9 wt. % carbon, 3.2 to 4.2wt. % silicon, 0.75 to 1.25 wt. % copper, 1.0 to 1.5 wt. % manganese,0.09 to 0.15 wt. % sulfur, and not greater than 0.2 wt. % phosphorous. Apiston ring formed of the cast iron is also provided.

Yet another aspect of the invention provides a method of manufacturing apiston ring formed of cast iron. The method includes providing an alloyincluding, in wt. % of the alloy, 2.2 to 2.9 wt. % carbon, 3.2 to 4.2wt. % silicon, 0.75 to 1.25 wt. % copper, 1.0 to 1.5 wt. % manganese,0.09 to 0.15 wt. % sulfur, and not greater than 0.1 wt. % phosphorous.The method then includes casting the alloy, austenitizing the alloy,quenching the autenitized alloy, and tempering the alloy. Another aspectof the invention provides a method of forming the cast iron.

The cast iron provides improved machinability, especially when used toform piston rings with complex design features, such as advanced oilgroove designs. The cast iron also provides exceptional bendingstrength, hardness, wear resistance, and elastic modulus. Further, thecast iron and piston ring formed of the cast iron are produced at lowmaterial and processing costs, compared to other methods used to formcast iron and piston rings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated,as the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawing wherein:

FIG. 1 is a perspective view of a piston ring according to oneembodiment of the invention.

DETAILED DESCRIPTION

One aspect of the invention provides an improved cast iron and a pistonring 20 formed of the cast iron, as generally shown in FIG. 1. The castiron provides improved machinability, so that complex physical featurescan be formed in the cast iron, such as advanced groove designs. Thecast iron also provides exceptional bending strength, hardness, wearresistance, and elastic modulus, at a minimum production cost. The castiron is formed by casting, austenitizing, quenching, and tempering analloy including specified amounts of carbon, silicon, copper, manganese,sulfur, and phosphorous. The resulting cast iron comprises a matrixhaving a microstructure including at least one of martensite andbainite. The cast iron also includes manganese sulfide (MnS) andcarbides dispersed throughout the matrix.

As shown in FIG. 1, piston ring 20 formed of the cast iron includes abottom surface 22 and an oppositely facing top surface 24 extendingparallel to one another and circumferentially around a center axis A.The piston ring 20 also presents an inner diameter surface 26 facingtoward the center axis A and an oppositely facing outer diameter surface28 each extending longitudinally from the bottom surface 22 to the topsurface 24. The outer diameter surface 28 and the inner diameter surface26 are typically parallel to one another and extend circumferentiallyaround the center axis A. During use of the piston ring 20 in an engineapplication, the outer diameter surface 28 contacts an inner surface ofa cylinder wall and provides a gas-tight seal therebetween. The pistonring 20 can include various design features formed therein, such as aplurality of oil grooves 30 along one or more of the surfaces 22, 24,26, 28. The piston ring 20 can include other surface modifications ordesigns, which depend on the application of the piston ring 20. Thepiston ring 20 can accommodate all surface modifications or designscommon to piston ring manufacturing.

The cast iron of the piston ring 20 includes specified amounts ofcarbon, silicon, copper, manganese, sulfur, and phosphorous, to providethe improved machinability and desirable physical properties. In oneembodiment, the cast iron includes carbon in an amount sufficient toprovide a carbon equivalent of 3.4 to 4.2, which is the carbonequivalent below the eutectic temperature of the cast iron, referred toas the average carbon equivalent hypoeutectic. The eutectic temperatureof the cast iron is typically 1145 to 1155° F. In one embodiment, thecast iron has an average carbon equivalent hypoeutectic of 3.8.

To provide the preferred carbon equivalent range of 3.4 to 4.2, the castiron typically includes, in weight percent (wt. %) of the cast iron, 2.2to 2.9 wt. % carbon, or 2.4 to 2.8 wt. % carbon, based on the totalweight of the cast iron. In another embodiment, the cast iron includesat least 2.2 wt. % carbon, or not greater than 2.9 wt. % carbon.

In one embodiment, the cast iron includes 3.2 to 4.2 wt. % silicon, or3.4 to 3.8 wt. % silicon. In another embodiment, the cast iron includesat least 3.2 wt. % silicon, or not greater than 4.2 wt. % silicon.

In one embodiment, the cast iron includes 0.75 to 1.25 wt. % copper, or0.8 to 1.0 wt. % copper. In another embodiment, the cast iron includesat least 0.75 wt. % copper, or not greater than 1.25 wt. % copper.

In one embodiment, the cast iron includes 1.0 to 1.5 wt. % manganese, or1.1 to 1.3 wt. % manganese. In another embodiment, the cast ironincludes at least 1.0 wt. % manganese, or not greater than 1.5 wt. %manganese.

In one embodiment, the cast iron includes 0.09 to 0.15 wt. % sulfur, or0.10 to 0.12 wt. % sulfur. In another embodiment, the cast iron includesat least 0.09 wt. % sulfur, or not greater than 0.15 wt. % sulfur.

In one embodiment, the cast iron includes not greater than 0.2 wt. %phosphorous, or 0.07 to 0.09 wt. % phosphorous. In another embodiment,the cast iron includes at least 0.01 wt. % phosphorous or not greaterthan 0.09 wt. % phosphorous.

The cast iron preferably includes chromium, vanadium, niobium, tungsten,and boron in amounts contributing to the exceptional wear resistance. Inone embodiment, the cast iron includes 0.25 to 0.65 wt. % chromium, or0.3 to 0.55 wt. % chromium. In another embodiment, the cast ironincludes at least 0.25 wt. % chromium, or not greater than 0.65 wt. %chromium. In one embodiment, the cast iron includes not greater than 0.2wt. % vanadium, or not greater than 0.17 wt. % vanadium. In anotherembodiment, the cast iron includes at least 0.01 wt. % vanadium. In oneembodiment, the cast iron includes not greater than 0.2 wt. % niobium,or not greater than 0.15 wt. % niobium. In another embodiment, the castiron includes at least 0.01 wt. % niobium. In one embodiment, the castiron includes not greater than 0.5 wt. % tungsten, or not greater than0.45 wt. % tungsten. In another embodiment, the cast iron includes atleast 0.01 wt. % tungsten. In one embodiment, the cast iron includes notgreater than 0.1 wt. % boron, or not greater than 0.08 wt. % boron. Inanother embodiment, the cast iron includes at least 0.01 wt. % boron.

The cast iron preferably includes molybdenum and nickel in an amountscontributing to the exceptional bending strength and hardness. In oneembodiment, the cast iron includes 0.3 to 0.5 wt. % molybdenum, or 0.34to 0.39 wt. % molybdenum. In another embodiment, the cast iron includesat least 0.3 wt. % molybdenum, or not greater than 0.5 wt. % molybdenum.In another embodiment, the cast iron includes not greater than 0.22 wt.% nickel, or 0.10 to 0.25 wt. % nickel. In yet another embodiment, thecast iron includes at least 0.10 wt. % nickel, or not greater than 0.25wt. % nickel.

The cast iron also preferably includes titanium in an amountcontributing to the exceptional physical properties. In one embodiment,the cast iron includes 0.03 to 0.09 wt. % titanium, or 0.04 to 0.08 wt.% titanium. In another embodiment, the cast iron includes at least 0.03wt. % titanium, or not greater than 0.09 wt. % titanium.

The remainder of the cast iron composition preferably consistsessentially of iron, and in an amount sufficient to provide the matrixincluding at least one of martensite and bainite. In one embodiment, thecast iron includes at least 75.0 wt. % iron, or at least 85.0 wt. %iron. Although the remainder of the cast iron preferably consists ofiron, the cast iron may also include impurities, preferably in an amountnot greater than 1.0 wt. %, based on the total weight of the cast iron.

The matrix of the cast iron includes a microstructure comprising atleast one of martensite and bainite, and preferably martensite. In oneembodiment, the matrix includes, in volume percent (vol. %) of thematrix, 80 to 90 vol. % martensite, based on the total volume of thematrix.

Preferably, the cast iron includes a fine distribution of carbidesdispersed throughout the matrix. The fine distribution of carbidescontributes to the exceptional wear resistance provided by the castiron. The amounts of chromium, vanadium, niobium, tungsten, and boroncontribute to the amount of carbides formed in the matrix. The cast ironalso includes manganese sulfide (MnS) dispersed throughout the matrix,which contributes to the improved machinability of the cast iron. Theamounts of manganese and sulfur contribute to the amount of MnS in thecast iron. In one embodiment, the cast iron includes 0.5 to 1.5 vol. %Mn, based on the total volume of the cast iron,

The matrix of the cast iron is preferably free of ferrite, austenite,and steadite. Preferably, the total amount of ferrite, austenite, andsteadite combined together is not greater than 5 vol. %, based on thetotal volume of the matrix. The low amounts of ferrite, austenite, andsteadite, and high amounts of martensite and bainite contributes to theimproved machinability and exceptional physical properties of the castiron.

The improvement in machinability of the cast iron allows for formationof the piston ring 20 with a variety of different designs along thesurfaces 22, 24, 26, 28 thereof. In one embodiment, the surfaces 22, 24,26, 28 of the piston ring 20 include at least one oil groove 30,protrusion, or other type of recesses, contour, or surface modification.

In addition to providing improved machinability, the cast iron providesexceptional bending strength, hardness, and elastic modulus. In oneembodiment, the cast iron has a bending strength of 750 to 1000 MPa, andtypically 780 to 850 MPa. The cast iron also typically has a BrinellHardness Number of 330 to 360 BHN, and a Rockwell Hardness B Scale valueof 100 to 116 HRB, typically 108 to 112 HRB. In one embodiment, the castiron has an elastic modulus of 115 to 160 GPa, typically 120 to 140 GPa.The cast iron also provides improved wear resistance. In one embodiment,the cast iron provides not greater than 70 microns of wear, typicallynot greater than 40 microns of wear, when tested using a Cameron-PlintModel TE-77 Reciprocating Sliding Wear Tester, according to ASTMStandard G-133.

Another aspect of the invention provides a method of forming theimproved cast iron and manufacturing the piston ring 20 formed of thecast iron. The method of forming the cast iron and piston ring 20 iseconomical and can be conducted at lower material and process costs,compared to other methods used to provide cast irons and piston rings.

The method first includes providing an iron alloy comprising, in weightpercent (wt. %) of the alloy: 2.2 to 2.9 wt. % carbon, or 2.4 to 2.8 wt.% carbon; 3.2 to 4.2 wt. % silicon, or 3.4 to 3.8 wt. % silicon; 0.75 to1.25 wt. % copper, or 0.8 to 1.0 wt. % copper; 1.0 to 1.5 wt. %manganese, or 1.1 to 1.3 wt. % manganese; 0.09 to 0.15 wt. % sulfur, or0.10 to 0.12 wt. % sulfur; 0.25 to 0.65 wt. % chromium, or 0.3 to 0.55wt. % chromium; 0.3 to 0.5 wt. % molybdenum, or 0.34 to 0.39 wt. %molybdenum; 0.03 to 0.09 wt. % titanium, or 0.04 to 0.08 wt. % titanium;not greater than 0.2 wt. % niobium, or not greater than 0.15 wt. %niobium; not greater than 0.5 wt. % tungsten, or not greater than 0.45wt. % tungsten; not greater than 0.1 wt. % boron, or not greater than0.08 wt. % boron; not greater than 0.2 wt. % vanadium, or not greaterthan 0.17 wt. % vanadium; not greater than 0.1 wt. % phosphorous, or0.07 to 0.09 wt. % phosphorous; not greater than 0.25 wt. % nickel, or0.10 to 0.25 wt. % nickel; and at least 75.0 wt. % iron, or at least85.0 wt. % iron.

The method next includes melting the alloy and casting the alloy in amold to provide a desired shape. In one embodiment, the mold providesthe shape of the piston ring 20 having the top surface 24, bottomsurface 22, inner diameter surface 26, and outer diameter surface 28.

Next, the method includes autenitizing the cast alloy by heating thecast alloy to a temperature of 1750 to 1875° F. for 60 to 120 minutes.Next, the method includes quenching the autenitized alloy in oil at atemperature of 140 to 150° F. The method further includes tempering thealloy after the quenching step at a temperature of 950 to 1150° F. for60 to 120 minutes to provide the finished cast iron.

Experiment

Performance tests were conducted for two cast irons formed according toembodiments of the invention. The compositions of the two inventiveexample cast irons, in wt. % of the cast iron, are disclosed in Table 1,as Examples 1 and 2. The inventive examples were compared to aconventional gray cast iron. The composition of the comparative graycast iron is also disclosed in Table 1.

TABLE 1 Inventive Inventive Comparative Example 1 Example 2 Example C2.68 2.67 3.38 Si 4.11 3.87 2.5 Cu 0.91 0.89 0.5 Mn 1.26 1.2 0.55 Cr0.38 0.52 0.23 V 0.03 0.16 0.05 Nb 0.01 0.11 0 B 0 0 0 W 0.01 0.26 0 P0.01 0.01 0.4 Ni 0.18 0.17 0 S 0.11 0.1 0.11 Mo 0.38 0.38 0 Ti 0.05 0.050 Fe remainder remainder remainder

The inventive and comparative cast irons were then tested for hardness,elastic modulus, bending strength, and wear resistance. The hardness ofeach cast iron was tested according to ASTM E18-086; Standard Method forRockwell hardness of metallic materials. The hardness test results areshown in Table 2 and illustrate the inventive cast irons have a greaterhardness than the conventional gray cast iron.

TABLE 2 Hardness (HRB) Inventive Inventive Comparative Example 1 Example2 Example 109.8 109.9 101.7

The inventive and comparative cast irons were next tested for elasticmodulus according to Federal-Mogul's Material Speciation for PistonRings, Designation GEO 504, found in the Federal-Mogul Piston RingHandbook, available athttp://www.federalmogul.com/korihandbook/en/index.htm. The elasticmodulus test results are shown in Table 3 and illustrate the inventivecast irons have a greater elastic modulus than the conventional graycast iron.

TABLE 3 Elastic Modulus (GPa) Inventive Inventive Comparative Example 1Example 2 Example 122 131 102

The inventive and comparative cast irons were next tested for bendingstrength according to Federal-Mogul's Material Speciation for PistonRings, Designation GEO 504, found in the Federal-Mogul Piston RingHandbook, available athttp://www.federalmogul.com/korihandbook/en/index.htm. The bendingstrength test results are shown in Table 4 and illustrate the inventivecast irons have a greater bending strength than the conventional graycast iron.

TABLE 4 Bending Strength (MPa) Inventive Inventive Comparative Example 1Example 2 Example 790 850 540

The inventive and comparative cast irons were next tested for wearresistance using a Cameron-Plint Model TE-77 Reciprocating Sliding WearTester, according to ASTM Standard G-133. The wear test results areshown in Table 5 and illustrate the inventive cast irons experiencesless wear than the conventional gray cast iron.

TABLE 5 Wear (μm) Inventive Inventive Comparative Example 1 Example 2Example 35 18 112

The inventive cast irons also provide advantages over other cast ironsof the prior art, such as those of U.S. Pat. Nos. 5,985,052; 5,972,128;and 4,891,076.

The cast iron of the '052 patent includes a matrix of acicular ferritein stable austenite with a mixture of non-broken, non-continuoussteadite, unlike the matrix of the inventive cast irons. The '052 patentmaterial also includes higher levels of phosphorous, which contributesto the high contents of steadite. The un-refined steadite of the '052patent cast iron leads to difficult machining, unlike the inventive castirons which provide exceptional machinability. The cast iron formedaccording to the '052 patent has a hardness of 280 to 330 BHN, which islower than the hardness of the inventive cast irons. Further, in thecast iron of the '052 patent is used to form a piston ring, thedecomposition of the austenite of the matrix during heating may revertthe matrix to a brittle martensite phase or softer phase during serviceof the piston ring in an internal combustion engine application.Further, the cast iron of the '052 patent is achieved by austempering,unlike the method of the present invention.

The cast iron of the '128 patent provides wear resistance usingexpensive alloys, and is more difficult to machine than the inventivecast iron. The hardness of the '128 patent material is 85 to 95 HRB,which is lower than the hardness of the invention cast irons. The castiron of the '076 patent is also achieved by austempering and includes amatrix of ferrite and austenite, unlike the cast iron according toembodiments of the present invention, which comprises a matrix includingat least one martensite or bainite with carbides and MnS dispersedthroughout the matrix.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings and may be practicedotherwise than as specifically described.

What is claimed is:
 1. A method of manufacturing a piston ring formed ofcast iron, comprising the steps of: providing an alloy including, inweight percent (wt. %) of the alloy, 2.2 to 2.9 wt. % carbon, 3.2 to 4.2wt. % silicon, 0.75 to 1.25 wt. % copper, 1.0 to 1.5 wt. % manganese,0.09 to 0.15 wt. % sulfur, and not greater than 0.1 wt. % phosphorous;casting the alloy; austenitizing the alloy; quenching the autenitizedalloy; and tempering the alloy.
 2. The method of claim 1 wherein theaustenitizing step includes heating the alloy to a temperature of 1750to 1850° F. for 60 to 120 minutes.
 3. The method of claim 1 wherein thequenching step includes disposing the austenitized alloy in oil at 140to 150° F.
 4. The method of claim 1 wherein the tempering step includesheating the quenched alloy to a temperature of 950 to 1000° F. for 60 to120 minutes.
 5. The method of claim 1 wherein the alloy includes, inweight percent (wt. %) of the alloy, 2.5 to 2.9 wt. % carbon, 3.6 to 4.2wt. % silicon, 0.75 to 1.25 wt. % copper, 1.0 to 1.5 wt. % manganese,and 0.09 to 0.15 wt. % sulfur.
 6. The method of claim 1 includingforming an oil grove in the cast iron.
 7. A method of forming a castiron, comprising the steps of: providing an alloy including, in weightpercent (wt. %) of the alloy, 2.2 to 2.9 wt. % carbon, 3.2 to 4.2 wt. %silicon, 0.75 to 1.25 wt. % copper, 1.0 to 1.5 wt. % manganese, 0.09 to0.15 wt. % sulfur, and not greater than 0.1 wt. % phosphorous; castingthe alloy; austenitizing the alloy; quenching the autenitized alloy; andtempering the alloy.